Lenslet array for beam homogenization

ABSTRACT

Apparatus for homogenizing a laser beam includes a lenslet array. In some embodiments, the lenslets have a negative power. The lenslet array may include from 16 to 36 effective lenslets in some embodiments, or any other suitable number in alternative embodiments. Some embodiments additionally include a re-focusing lens for directing the beamlets onto a target so that the beamlets overlap and the energy distribution is homogenized. In an alternative embodiment, the lenslet array and re-focusing lens are combined in one optic.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/683,963 filed Mar. 8, 2007, which application is a continuation ofU.S. patent application Ser. No. 10/913,952 filed Aug. 6, 2004 (now U.S.Pat. No. 7,206,132); the full disclosures of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to laser beam delivery systems.More specifically, the invention relates to devices, systems and methodsfor homogenizing a laser beam for use in refractive surgery.

Laser beam delivery systems designed to improve the temporal and spatialcharacteristics of collimated beams of radiation with non-symmetricalenergy profile cross sections are known. Some systems, for example, areused to deliver excimer laser beams for performing refractive surgery.In the STAR™ System, developed by VISX, Incorporated (Santa Clara,Calif.), a collimated laser beam used for photorefractive keratectomy(PRK) and phototherapeutic keratectomy (PTK) is delivered to the planeof surgery by means of an optical beam delivery system which providesboth spatial and temporal integration for an excimer laser beam. In thissystem, a collimated laser beam is passed through a set of six prismssurrounding an open path to divide the incoming beam into sevenbeamlets. Further averaging in the temporal domain is performed byrotating the beam with a rotating telescope. The combination of beamsplitting and rotation produces a laser beam having an intensity profilethat may be used for refractive surgery. Such a system is described inU.S. Pat. Nos. 5,646,791 and 5,912,775, which are assigned to theassignee of the present invention and which are hereby incorporatedfully by reference.

While highly effective in providing spatial and temporal integration toa collimated laser beam, this arrangement sometimes provides beamletswith minor non-uniformities, thus resulting in a laser beam having aslightly varied cross-sectional intensity profile at an ablation target.In other words, such an arrangement may provide less laser beamhomogenization (or intensity profile averaging) than would be optimal.One solution would be to increase the number of prisms in thehomogenization device, but such a device would be difficult tomanufacture. Additionally, transmission of optics exposed to excimerlasers deteriorates with time, and this effect is especially large inrelatively thick prism elements. Another disadvantage of a system asdescribed in U.S. Pat. Nos. 5,646,791 and 5,912,775 is that alignment ofthe system can be relatively challenging.

Therefore, a need exists for improved laser beam homogenizing devices,systems and methods. Such devices, systems and methods would ideallyprovide for enhanced laser beam intensity averaging and homogenization,thus reducing or eliminating variations in intensity over across-section of a laser beam. Ideally, beamlets in such a system wouldbe collimated. Also ideally, devices for homogenizing a laser beam wouldbe relatively simple to produce, would provide for enhanced transmissionof light and would be relatively resistant to wear and tear. At leastsome of these objectives will be met by the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention generally provides devices, systems and methodsincluding a lenslet array for homogenizing a laser beam. In one aspectof the invention, apparatus for altering an energy distribution across alaser beam comprises an array of negative power lenslets. In someembodiments, for example, the lenslet array comprises a square grid ofat least 16 lenslets at least partially within the beam. For example, insome embodiments each lenslet of the lenslet array has a cross-sectionaldimension of between about 2 mm and about 5 mm. The lenslet array mayhave any suitable shape or configuration, but in one embodiment thearray comprises a hexagonal grid. Although any other suitable materialmay be used, in one embodiment the lenslet array comprises fused silica.

In some embodiments, the lenslet array includes a first side comprisinga first linear array of concave cylindrical surfaces and a second sideopposite the first side and comprising a second linear array of concavecylindrical surfaces extending perpendicular to the surfaces of firstlinear array. Some embodiments further include a drive for rotating thelenslet array about a longitudinal axis extending along the laser beam.

In another aspect of the invention, apparatus for homogenizing an energydistribution across a laser beam includes a lenslet array fortransmitting the laser beam as multiple beamlets, each lenslet having aneffective negative power and at least one re-focusing lens for directingthe beamlets onto a target so that the beamlets overlap and the energydistribution is homogenized. Optionally, the apparatus may furtherinclude at least one rotating member for rotating the lenslet arrayabout a longitudinal axis of the laser beam. In some embodiments, thelenslet array comprises multiple negative power lenslets. In variousembodiments, the lenslet array may have any of the features describedabove.

In some embodiments, the lenslets are rotationally offset between firinglaser pulses to account for coupling effects between a laser source anda geometry of the array. In some embodiments, the lenslet arraycomprises fused silica. Also in some embodiments, the lenslet arraycomprises a first side comprising a first linear array of concavecylindrical surfaces and a second side opposite the first side andcomprising a second linear array of concave cylindrical surfacesextending perpendicular to the surfaces of first linear array. In someembodiments, the re-focusing lens comprises a positive power lens.

Optionally, the apparatus may further include a drive for rotating thelens about a longitudinal axis extending along the laser beam. In someembodiments, the apparatus also includes an aperture disposed at a planewhere the combined beamlets overlay to size a beam passing through theaperture. Optionally, the apparatus may further include a telescope toadjust a cross-sectional area of the laser beam before the laser beamsarrives at the lenslet array. In some embodiments, the telescope has afixed position relative to the laser beam.

In another aspect of the present invention, a system for providing alaser beam having a homogenized energy distribution to an eye of apatient includes a source of laser energy, a lenslet array fortransmitting the laser beam as multiple beamlets, each lenslet having aneffective negative power, and at least one re-focusing lens fordirecting the beamlets onto a target so that the beamlets overlap andthe energy distribution is homogenized. The lenslet array andre-focusing lens may include any of the features described above.

Optionally, the system may further include a drive for rotating thelenslet array about a longitudinal axis extending along the laser beam.The system may further include an aperture disposed at a plane where thecombined beamlets overlap to size a beam passing through the aperture.In some embodiments, the system includes a telescope to adjust across-sectional area of the laser beam upstream of the lenslet array.

In another aspect of the present invention, a system for providing alaser beam having a homogenized energy distribution to an eye of apatient includes a laser providing a laser beam having unequaldivergence in two perpendicular axes, a negative powered lenslet arrayfor transmitting the laser beam as multiple beamlets, the lenslet arraycomprising opposed surfaces of crossed concave cylindrical surfaces, adrive for rotating the lenslet array about a longitudinal axis extendingalong the laser beam, and at least one lens for directing the beamletsonto a target so that the beamlets overlap and the energy distributionis homogenized. In this aspect of the invention, the system isconfigured to fire the laser when the lenslet array is rotated away from90° and 0° for an excimer laser. The excimer laser has an asymmetricalbeam shape, which can be used to define an axis of rotation.

In another aspect of the present invention, a method for homogenizing anenergy distribution across a laser beam includes passing a laser beamthrough a lenslet array to transmit the laser beam as multiple beamletsand directing the beamlets onto a target using at least one lens, sothat the beamlets overlap and the energy distribution is homogenized. Insome embodiments, the lenslet array comprises a negative power lensletarray, and passing the beam through the lenslet array forms divergingbeamlets. Some embodiments involve passing the laser beam through thelenslet array to generate at least 16 beamlets.

Optionally, the method may further include rotating the lenslet arrayabout a longitudinal axis extending along the laser beam. In someembodiments, for example, the lenslet array rotates such that each pulseof a pulsed laser beam passes through the array in an angular window ofacceptance about 45° diagonal axes, thus avoiding 0° and 90°orientations of the array relative to the laser beam. In one embodiment,the angular window of acceptance comprises a range of 10° on both sidesof the 45° diagonal axes. The method may optionally further includerotating the lens about the longitudinal axis.

In some embodiments, the method further involves adjusting across-sectional dimension of the laser beam by passing the beam througha telescope before passing the beam through the lenslet array. In someembodiments, directing the beamlets onto a target involves focusing thebeamlets on an aperture disposed apart from the lens. Optionally, themethod may further involve directing at least some of the beamletsthrough an aperture. In some embodiments, directing the beamlets throughthe aperture may cause the beamlets to arrive collimated at an ablationplane. Also in some embodiments, a cross-sectional dimension of theaperture is selected to provide the laser beam with a desiredcross-sectional dimension at an ablation plane.

In another aspect of the present invention, a method of providing alaser beam having a homogenized energy distribution to an eye of apatient involves passing a laser beam through a lenslet array totransmit the laser beam as multiple beamlets, directing the beamletsthrough at least one lens, so that the beamlets overlap and the energydistribution is homogenized, and directing at least some of the beamletsthrough an aperture. Various embodiments of this method may include anyof the features described above.

These and other aspects of the invention are described in greater detailbelow, with reference to the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side-view diagram of a portion of a laser beamoptical delivery system incorporating one embodiment of the invention;

FIG. 1A is an end-on view of a lenslet array according to one embodimentof the present invention; is a schematic sectional view taken alonglines 2-2 of FIG. 1 of a portion of the spatial beam integrator;

FIGS. 2A and 2B are perspective views of a lenslet array comprisingopposed surfaces according to one embodiment of the present invention;and

FIG. 3 is a schematic diagram of a laser beam optical delivery systemincorporating one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, FIG. 1 illustrates in schematic form alaser beam delivery apparatus according to the invention. As seen inthis figure, a collimated beam 10 from a laser source (not shown) isdirected onto the inlet face of a lenslet array 12. Lenslet array 12divides the beam 12 into multiple beamlets 14, which then pass through are-focusing lens 16. The refocused beamlets 18 then pass through anaperture 20, adapted for sizing the beamlets and providing a desiredcross-sectional beam profile.

Referring to FIG. 1A, the lenslet array 12 may have any suitable numberand configuration of lenslets 13. In some embodiments, the lenslet array12 comprises a square grid of lenslets 13, as shown, with at least 16lenslets configured to be disposed within the path of a laser beam 10.In other embodiments, however, any other suitable number of lenslets 13may be included and disposed such that the laser beam 10 passes throughall or a portion of them. Furthermore, the size, geometry andconfiguration of the lenslets 13, and/or 12 the lenslet array 12 mayvary in different embodiments. In one embodiment, for example, thelenslet array 12 and the lenslets 13 may both be hexagonal. In anotherembodiment, as described further below in reference to FIGS. 2A and 2B,the lenslet array 12 may comprise two opposing surfaces of multiplehalf-cylinders. Lenslets 13 may also have any suitable optical power(focal length). The characteristics of size, shape, geometry, opticalpower and the like of both the individual lenslets 13 and the lensletarray 12 may be selected, in various embodiments, to provide a desiredbeam profile at the ablation plane. In one embodiment, for example, thelenslet array 12 includes 16 effective lenslets 13, arranged in a 4×4square grid, and each lens is about 4.5 mm square, with a focal lengthof about f=−123.5 mm. Other embodiments may include from 16 to 36effective lenslets, or any other suitable number.

The beamlets 14 are combined by means of tilting optics, such as theconvex refocusing lens 16, or in alternative embodiments a concavemirror, or other optic element(s). The aperture 20 is positioned at aplane where the beamlets 18 overlap and is adapted to size the beam anddiscard undesired intensity variation at the edges of the beam. Thisselectable aperture 20 is then optically imaged onto, or close to, theablation plane. The size and geometry of the imaged spot at the pointwhere the beamlets 18 overlap is determined by the selection aperture 20and magnification of the lens 16. The overlap area at the selectionaperture 20 depends on the size of each lenslet 13 and the lens power ofthe re-focusing lens 16, but is limited in uniformity by the divergencein each beamlet 18 at that plane. The divergence of a beamlet 18 isdetermined by the power of a lenslet 13 and the effective power of thelens 16. Incoming divergence from the laser source may also beconsidered. While a larger number of lenslets 13 implies better spatialaveraging, smaller lenslets 13 also cause higher divergence, thuscausing a laser beam profile that is not as uniform and more peaked inshape. In order to fill a desired size of the aperture 20 and stillachieve a uniform intensity profile, the size and power of the lenslets13 may be chosen so as to achieve a compromise between spatial averagingand beamlet divergence.

The lenslet array 12 may be either positive or negative power. Thebeamlets emitted through a positive power lenslet 12 are converging,while beamlets emitted through a negative power lenslet array 12 arediverging. It may be advantageous to use a negative power lenslet array12 in some embodiments, in order to avoid unwanted beamlet focusing andresulting “hot spots,” where laser fluence is high and excess ozone iscreated if the laser is an Argon Fluoride Excimer laser.

With reference now to FIGS. 2A and 2B, in one embodiment a lenslet array20 comprises two opposed surfaces 22 a, 22 b, each surface includingmultiple partial cylinders 24, arranged in parallel to form multipleconcavities. The partial cylinders 24 on one surface 22 a, are arrangedapproximately orthogonally relative to the partial cylinders 24 on theopposed surface 22 b. It has been found that opposed surfaces 22 a, 22 bwith oppositely oriented partial cylinders 24 function as lenslets, suchas multiple spherical lenslets. Furthermore, a lenslet array 20 as shownis typically easier to manufacture than an alternative array comprisingmultiple spherical lenslets. For example, the array 20 may be made offused silica, with the concave partial cylinders 24 etched on thesurfaces 22 a, 22 b.

Referring now to FIG. 3, in one embodiment, a laser beam 30 is providedby a laser source 32 and directed toward a first mirror 33. The mirror33 directs the beam 30 toward a sizing telescope 34, which sizes thebeam 30 to a desired cross-sectional size. In one embodiment, forexample, the cross sectional area of the sized beam 30 may be about 18mm by about 20 mm. A lenslet aperture 36 is placed before the lensletarray 38 to size the beam 30 again, such as to create an 18 mm by 18 mmbeam in one embodiment. The aperture 36 also aligned the beam 30 to thelenslet array 38 so as to fill the 16-lenslet, 4×4 grid. A positivepower re-focusing lens 40 is placed after the lenslet array 38 to tiltthe beamlets formed by the array 38 and to overlap the beamlets at aniris aperture 42, where the beamlets arrive after contacting anothermirror 41. The beamlets may then pass through additional imaging lenses44, 48 and mirrors 45, 46, 49, before reaching an ablation plane 50.

In one embodiment, the lenslet array 38 may be rotated about an axisbetween pulses of the laser beam 30. For example, the array 38 may berotated approximately 45° away from the 0° and 90° angles relative tothe axis of the laser beam 30. In some embodiments, the angles ofrotation of the array 38 are within a window of 45°+/−10°, meaningbetween 35° and 55°. Rotation may occur in 90° increments between eachpulse of a pulsed laser source 32, and with each rotation being within awindow +/−10°. For example, angles of rotation could be about 45°, 135°,225°, and 315°. Such rotation helps prevent formation of a laser beamhaving areas of striping or a grid pattern caused by the areas of thelenslet array 38 between lenslets. Rotation may be achieved via a drivemechanism coupled with the lenslet array 38 or via any other suitablemeans.

In some embodiments, the lenslet array 38 and the re-focusing lens 40may be combined in one optic. This may be achieved, for example, by alenslet array 38 having a gradual curvature along one or both of itsopposed surfaces to give the lenslet array 38 an overall low positivepower lens effect. Such a combined optic is adapted to split the laserbeam 30 into multiple beamlets and also make the beamlets overlap in theplane of the aperture 42. In various embodiments, such a curved lensletarray 38 could be formed by programming lithography software to give aslight curvature to the optic or by disposing the partial cylindricalelements of the lenslet 38 on a curved substrate.

While the above provides a full and complete disclosure of the preferredembodiments of the invention, various modifications, alternateconstructions and equivalents will occur to those skilled in the art.For example, while the invention has been described with expressreference to an ophthalmological laser surgery system, otherapplications of the invention may be made, as desired. Therefore, theabove should not be construed as limiting the invention, which isdefined by the appended claims.

1. Apparatus for altering an energy distribution across a laser beam,the apparatus comprising: an array of optical power lenslets arranged ina pattern; and a structure to support the lenslet array with the lensletarray rotated in relation to two axes of the laser beam having anunequal divergence to account for coupling effects between the gridpattern and the two axes of the laser beam.
 2. Apparatus as in claim 1,wherein the lenslet array comprises a square grid of at least 16lenslets at least partially within the beam.
 3. Apparatus as in claim 1,wherein the lenslet array comprises a hexagonal grid.
 4. Apparatus as inclaim 1, wherein the lenslet array comprises fused silica.
 5. Apparatusas in claim 1, wherein the lenslet array comprises: a first sidecomprising a first linear array of concave cylindrical surfaces; and asecond side opposite the first side and comprising a second linear arrayof concave cylindrical surfaces extending perpendicular to the surfacesof first linear array.
 6. Apparatus as in claim 1, further comprising adrive for rotating the lenslet array about a longitudinal axis extendingalong the laser beam.
 7. Apparatus as in claim 1, further comprising are-focusing lens for directing the beamlets onto a target so that thebeamlets overlap and the energy distribution is homogenized. 8.Apparatus for homogenizing an energy distribution across a laser beam,the apparatus comprising: a lenslet array for transmitting the laserbeam as multiple beamlets, each lenslet having an effective opticalpower, wherein the lenslets are rotationally offset to account forcoupling effects between a laser source and a geometry of the array. 9.Apparatus as in claim 8, further comprising at least one re-focusinglens for directing the beamlets onto a target so that the beamletsoverlap and the energy distribution is homogenized.
 10. Apparatus as inclaim 9, wherein the lenslet array and the re-focusing lens are combinedin one optic.
 11. Apparatus as in claim 8, further comprising at leastone rotating member for rotating the lenslet array about a longitudinalaxis of the laser beam.
 12. A system for providing a laser beam having ahomogenized energy distribution to an eye of a patient, the systemcomprising: a source of laser energy to provide a pulsed laser beam withan unequal divergence in two axes; a lenslet array to transmit the laserbeam as multiple beamlets, each lenslet having an effective opticalpower, the lenslet array comprising a pattern; and a mechanism to rotatethe lenslet array about a longitudinal axis extending along the laserbeam to rotationally offset the lenslet array and account for couplingof the unequal divergence in two axes of the laser beam with the patternof the array.
 13. A system as in claim 12, further comprising at leastone focusing lens to direct the beamlets onto a target so that thebeamlets overlap and the energy distribution is homogenized.
 14. Asystem as in claim 12, wherein the system is configured to fire thelaser when the lenslet array is rotationally offset.
 15. A method forhomogenizing an energy distribution across a laser beam, the methodcomprising: generating the laser beam with a laser, the laser beamcomprising an unequal divergence in two axes; and passing a laser beamthrough a lenslet array having a pattern to transmit the laser beam asmultiple beamlets, wherein the lenslet array is rotated about alongitudinal axis extending along the laser beam to account for couplingbetween the pattern and the unequal divergence of the two axes.
 16. Amethod as in claim 15, further comprising directing the beamlets onto atarget using at least one lens, so that the beamlets overlap and theenergy distribution is homogenized.
 17. A method as in claim 15, whereinthe lenslet array comprises a negative power lenslet array, and whereinpassing the beam through the lenslet array forms diverging beamlets. 18.A method as in claim 15, further comprising directing at least some ofthe beamlets through an aperture.
 19. A method as in claim 18, whereindirecting the beamlets through the aperture causes the beamlets toarrive collimated at an ablation plane.
 20. A method as in claim 18,wherein a cross-sectional dimension of the aperture is selected toprovide the laser beam with a desired cross-sectional dimension at anablation plane.